1. Field of the Invention
This invention relates to a fuel supply control system for an internal combustion engine, which controls the supply and cutoff of fuel to the engine based on the amount of oxygen stored in a catalytic converter which purifies exhaust gases emitted from the engine.
2. Description of the Prior Art
The present assignee proposed an air-fuel ratio control system which controls the air-fuel ratio of an air-fuel mixture to be supplied to an internal combustion engine, based on the amount of oxygen stored in the above catalytic converter, e.g. in Japanese Patent Application No. 5-329780 (corresponding to Japanese Laid-Open Patent Publication (Kokai) No. 7-151002), and a fuel supply control system which carries out the cutoff of fuel (fuel cutoff) to an internal combustion engine during deceleration of the engine, e.g. in Japanese Patent Application No. 7-270736 (corresponding to Japanese Laid-Open Patent Publication (Kokai) No. 9-86227).
In the above air-fuel ratio control system, two O2 sensors (oxygen sensors) are arranged at locations upstream and downstream of a catalytic converter in an exhaust pipe, for detecting the concentration of oxygen in exhaust gases. The amount of oxygen stored in the catalytic converter is estimated based on results of detection performed by the O2 sensors. Then, a desired air-fuel ratio is calculated in dependence on the estimated oxygen storage amount, and the air-fuel ratio of the air-fuel mixture is feedback-controlled such that the air-fuel ratio becomes equal to the desired air-fuel ratio. This makes it possible to control the air-fuel ratio such that the purification rate of the catalytic converter is maximized. On the other hand, in the fuel supply control system, to enhance drivability, fuel cutoff is executed during deceleration of the engine, after a predetermined time period has elapsed from a time point the conditions for carrying out the fuel cutoff were fulfilled. Particularly when deceleration shift is being carried out, the above predetermined time period is shortened to thereby carry out the fuel cutoff promptly after the conditions are fulfilled.
The air-fuel ratio control carried out by the air-fuel ratio control system and the fuel cutoff control executed by the fuel supply control system can attain their respective goals. However, they are carried out separately and independently. Therefore, for instance, when the amount of oxygen stored in the catalytic converter is considered to be large and accordingly the air-fuel ratio is controlled to be richer than a stoichiometric air-fuel ratio, if fuel cutoff is executed, the amount of oxygen stored in the catalytic converter (oxygen storage amount) is further increased, which results in a degraded purification rate of the catalytic converter.
It is an object of the invention to provide a fuel supply control system for an internal combustion engine, which is capable of controlling fuel cutoff according to the amount of oxygen stored in exhaust gas purification means to thereby enhance the purification rate of the exhaust gas purification means while maintaining excellent fuel economy, thereby making it possible to improve exhaust emission characteristics.
To attain the above object, the present invention provides a fuel supply control system for an internal combustion engine having an exhaust system, for controlling supply of fuel to the engine, comprising:
exhaust gas purification means arranged in the exhaust system of the engine;
oxygen storage amount estimation means for estimating an amount of oxygen stored in the exhaust gas purification means, as an oxygen storage amount;
deceleration condition-detecting means for detecting a deceleration condition of the engine;
fuel supply cutoff means for cutting off the supply of the fuel to the engine when the deceleration condition-detecting means has detected the deceleration condition; and
control means for controlling the fuel supply cutoff means based on the oxygen storage amount estimated by the oxygen storage amount estimation means.
According to this fuel supply control system, the fuel supply cutoff means which cuts off the supply of fuel to the engine when the deceleration condition of the internal combustion engine has been detected is controlled based on the amount of oxygen stored in the exhaust gas purification means, which is estimated by the oxygen storage amount estimation means. As described above, the cutoff of supply of fuel to the engine (fuel cutoff) by the fuel supply cutoff means is controlled based on the oxygen storage amount, whereby it is possible to enhance the purification rate of the exhaust gas purification means while maintaining excellent fuel economy. This results in improved exhaust emission characteristics. For instance, when the estimated oxygen storage amount is small, a time period (hereinafter referred to as xe2x80x9cthe delay timexe2x80x9d throughout the specification) between a time point conditions for carrying out fuel cutoff are fulfilled and a time point the fuel cutoff starts to be actually executed is shortened to carry out the fuel cutoff promptly, allowing the oxygen storage amount to be increased. On the other hand, when the estimated oxygen storage amount is large, the delay time is increased to delay execution of the fuel cutoff, thereby making it possible to prevent the oxygen storage amount from being increased. Further, when the oxygen storage amount is increased to a certain amount, the fuel cutoff being performed may be interrupted, thereby making it possible to prevent the oxygen storage amount from being increased to an extremely large amount. As described above, positive use of fuel cutoff is made during deceleration of the engine, whereby it is possible to control an actual amount of oxygen stored in the exhaust gas purification means. This makes it possible to enhance the purification rate of the exhaust gas purification means while maintaining excellent fuel economy.
Preferably, the fuel supply control system includes fuel cutoff inhibition means for inhibiting the fuel supply cutoff means from cutting off the supply of the fuel to the engine, when the oxygen storage amount estimated by the oxygen storage amount estimation means is larger than a predetermined maximum storage amount.
Preferably, the fuel supply control system includes delay time-setting means for setting a delay time over which execution of the cutoff of the supply of the fuel to the engine is delayed, according to the oxygen storage amount.
Preferably, the fuel supply control system includes engine rotational speed-detecting means for detecting a rotational speed of the engine, and intake pipe absolute pressure-detecting means for detecting an intake pipe absolute pressure, and the oxygen storage amount estimation means estimates the oxygen storage amount by adding or subtracting an incremental/decremental value calculated based on a space velocity representative of a volume of exhaust gases, to or from an immediately preceding value of the oxygen storage amount, in accordance with a state of fuel supply control, the space velocity being calculated by using a product of a value of the engine rotational speed detected by the engine rotational speed-detecting means and a value of the intake pipe absolute pressure detected by the intake pipe absolute pressure-detecting means.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.